Basic Electricity question network.. Ra=R1 x R2?

Hi, I am taking a basic electricity class and I am confused on something. I think it's called a Network, but I'm not exactly sure. The teacher isn't going by the textbook and I can't find anything in the book that would explain.

This is what I wrote down Ra= R1x R3
R1+R2+R3

What is this called so I can look it up? Also, are there any websites where I can practice problems? My book has some problems, but it doesn't give the answer, so I'm not sure if I am doing them right. Thank you for any help you can give.